1. Field of the Invention
The present invention relates to a laser annealing method, more particularly, to a laser annealing method for crystallizing a silicon film.
2. Discussion of the Related Art
Laser annealing is used for activating an amorphous part of a silicon film pattern by impurity doping or crystallizing a silicon film. Generally, silicon film crystallization by laser is carried out by supplying a film with laser energy by scanning a silicon film with a laser beam having a predetermined profile by overlapping.
FIG. 1 shows a graph of laser energy density vs. size of crystal particle when an amorphous silicon film is irradiated with an excimer laser by a single shot.
Referring to FIG. 1, in region I where energy is under a predetermined value, the size of laser particle increase gradually even though the laser energy density increases. In region II where energy density is over the predetermined value, crystal particle size increases abruptly. In region III where energy density is larger than that of region II, the crystal particle size is reduced abruptly. In particular, after the amorphous silicon film has been melted down, the crystal particle size is changed greatly in accordance with laser energy density.
The respective regions are explained in more detail in the following description.
In region I, an upper part of the amorphous silicon film is melted partially due to low laser energy density. In this case, crystal particles start to grow as the melted upper silicon proceeds to be solidified from the fine crystallite seeds in the lower amorphous silicon film. Since the crystal particles grow vertically, the crystal particle size changes less and is small with respect to energy density variation.
In region II, melting continues to an interface and unmelted silicon particles exist in a portion of the interface. In this case, the crystal particle size increases greatly since the crystal particles grow laterally to the direction of the melted silicon by using the remaining silicon crystal particles which have not yet been melted as seeds. As the density of the seeds becomes smaller, the crystal particle size becomes larger. In this case, the crystal particle size may increase 10 times larger than that in region I. However, as can be seen in the drawing, the variation of the crystal particle size in this region fluctuates greatly in accordance with the laser energy density. Namely, a process window is very small since there is a high variation of the crystal particle size with respect to a minute variation of the energy density. Accordingly, when laser crystallization is carried out within region II, the maintenance of process equipments is very difficult due to a narrow process window. Moreover, this causes low yield which makes it disadvantageous for mass production.
In region III, there is no remaining crystal particle in the interface since the amorphous silicon film including the interface is completely melted down. In this case, during the solidification of the melted silicon, nuclei are generated to grow frequently and simultaneously, thereby forming fine grains.
FIGS. 2A to 2C show schematic layouts of silicon crystallization by laser beam irradiation to explain laser annealing according to a related art. Here, a simplified figure of silicon is shown in the drawing. The figure or shape of a laser beam for laser annealing is patterned by an optical system, thereby preparing a line type figure. In this case, the laser beam is patterned to have a width between about 0.1.about.2 mm and a length between about 10.about.1000 mm and its energy density has a range sufficient to melt an amorphous silicon film.
Referring to FIG. 2A, a first irradiation is carried out on a silicon film with a laser beam adjusted to have the above-mentioned conditions by an optical system. A portion of the first irradiated amorphous silicon is melted to an interface. Yet, unmelted silicon particles still exist at the interface. Crystallization takes place in the direction of the melted silicon since the particles working as seeds for crystallization grow laterally. As a result, a polycrystalline silicon region consisting of a plurality of silicon grains is produced.
Referring to FIG. 2B, having been displaced to a predetermined distance, the silicon film undergoes a second irradiation with the laser beam. The displacement of the substrate is related to the overlap degree of the laser beam. Thus, as the displacement of the substrate becomes longer, the degree of overlap becomes smaller. Generally, in order to grow a silicon grain up to 4000.about.5000 .ANG., the silicon film has to be irradiated with a laser beam 10 to 20 times repeatedly to provide sufficient laser energy with the irradiations being carried out overlapping one another. In general, the overlap degree of the laser beam lies between about 80.about.99%. A plurality of silicon grains 10 to 10,000 .ANG. long exists in the silicon film crystallized by a single shot of a laser beam. Small silicon grains are recrystallized to grow provided that the same spot of such polycrystalline silicon firm undergoes laser beam irradiations more than several times. This results in the formation of uniformly large silicon grains at a certain time. Therefore, a specific portion of a silicon film needs to be irradiated with laser beams several times to form large silicon grains uniformly in the overall film during the silicon film crystallization.
In FIG. 2B, 21 designates a displacement and a direction of relative movement of a laser beam against the silicon film is shown by arrows. A portion of the second irradiated amorphous silicon is melted down again to grow and be crystallized. In this case, silicon grains in the portion irradiated with the laser beam repeatedly become larger. G1 denotes the silicon grains irradiated by the laser beam once, while G2 designates other silicon grains irradiated by the laser beams twice. The entire amorphous silicon film is irradiated with the laser beams repeatedly for crystallization by scanning the entire surface of the substrate.
Referring to FIG. 2C, a polycrystalline silicon film consisting of silicon grains G3 grown to 4000.about.5000 .ANG. is produced. In accordance with the above description, the silicon grains G3 shown in FIG. 2C are formed by being supplied with laser energy repeatedly 10 to 20 times.
However, the related art has several problems. The related art requires increased process time due to many repeated irradiations of laser beams on a silicon film of a predetermined area needing a high degree of overlap of the laser beams. Also, the laser annealing process is carried out in a vacuum to reduce surface roughness due to high grain boundaries resulting from multiple laser beam irradiations. Thus, the related art requires a special annealing apparatus to meet the need for a vacuum atmosphere. The related art uses a small energy region with a small process window which makes it difficult to construct a proper optical system. Moreover, such problems lead to reduced productivity and low product yield.